Effect and mechanism analysis of MnO2 on permeable reactive barrier (PRB) system for the removal of tetracycline.

Effect of manganese dioxide (MnO2) on tetracycline (TC) removal/degradation in zero-valent iron (ZVI) based permeable reactive barrier (PRB) system was investigated. To analyze the role of MnO2, three different PRB columns packed with ZVI, ZVI and a layer of MnO2, and MnO2 were set up to investigate the removal effect and reaction mechanism of ZVI coupling with MnO2 on TC removal, respectively. The results show that the removal efficiencies of three PRB columns are 65%, 85%, and 50%, respectively. MnO2 could accelerate the transformation of Fe2+ into Fe3+ and combine with Fe3+ to degrade TC in different reaction sites in the ZVI-MnO2 PRB system. Hydroxyl radicals (·OH) were produced in this process, which contributed to about 58.3% for the TC degradation. The UV-Vis spectrum demonstrated that A ring of TC was the main reaction site for interaction with Fe3+ and the BCD rings were crucial for interactions with MnO2. On the basis of intermediates identified by LC-ESI-MS, the ring structure of TC was opened, and low-molecular-weight compounds were produced in ZVI-MnO2 PRB system.